A data-rewritable optical disc such as CD-RW (Compact Disc-Rewritable) is widely used to record data so the data can be amended when necessary. Recently, CD-MRW (CD-Mount Rainier Written) has been introduced to the market. Such type of data-rewritable optical disc is especially advantageous for providing background formatting and defect management to examine and prepare the recordable area of a disc. According to CD-MRW technique, defective blocks will be marked and managed so that these defective blocks will be hidden and not available for further accessing.
An approach to detect defective blocks will be described herein with reference to FIGS. 1 and 2. The data processing system of FIG. 1 comprises an optical pickup head 20, a pre-amplifier 21, a sub-beam addition signal (SBAD) low pass filter 22, a digital signal processor (DSP) 23 and a micro-controller 24. The optical pickup head 20 has several light receiving parts (not shown) for respectively receiving the light reflected from one of the blocks of a disc 10, e.g. block 11, and producing sub-beam signals SB (Step S11, FIG. 2). These sub-beam signals SB are amplified through the pre-amplifier 21 to generate a sub-beam addition signal SBAD (Step S12). The sub-beam addition signal SBAD is substantially the summation partial or all of these sub-beam signals. The sub-beam addition signal SBAD is then filtered by the SBAD low pass filter 22 to generate a low pass signal SBADlow pass (Step S13). If an absolute value of |SBAD−SBADlow pass| is greater than a threshold value, the block is determined to be a defective block (Step S14), or otherwise, a non-defective block (Step S15). The information relating to the defective block is then recorded in the micro-controller 24 so as to prevent from writing data into the defective block subsequently (Step S16). The above detecting steps are repeated until all blocks on the disc 10 are tested.
The above method for detecting defective blocks, however, may have some limitation due to improper threshold setting or noise interference. For example, as shown in FIG. 3, a scratch S and some defective blocks, e.g. C and D, exist in the optical disc 30. Under certain high threshold or noise condition, it is possible that only blocks A and B in the scratch S are detected as defective blocks according to the above defect-detection method. Since the blocks included in the scratch S, except the blocks A and B, are determined as non-defective ones, data will be subsequently written therein. Thus, accessing errors may still occur on the undetected but damaged blocks in the vicinity of the scratch S other than block A and B.